1. Field of the Invention
The present invention relates to electrical insulation materials which are suitable for high voltage applications. In particular, the invention relates to additives for polymeric insulation materials, especially for enhancing the water repellent properties of such materials and/or for suppressing the occurrence of leakage currents on such materials at high voltages.
2. Introduction to the Invention
In high voltage applications (e.g. 1 kV or above), the condensation of water on the surface of polymeric insulation material can result in leakage currents flowing across the surface of the insulation material. These leakage currents normally cause the temperature of the insulation material to rise, leading to evaporation of the moisture and the formation of so-called dry bands. This in turn generally leads to electrical discharges on the surface of the insulation. This electrical discharge activity produces corrosive by-products such as ozone, nitrogen oxides, sulphur dioxide, and ultraviolet radiation. If the insulation material is exposed to the outdoors environment, such by-products may be blown or washed away, and they may therefore cause little or no damage to the polymeric insulation material. However, if the insulation material is protected to some degree from the external environment (for example if it is enclosed in a cabinet) the by-products of the electrical discharge activity may remain and attack the surface of the polymeric insulation material. Electrochemical processes occurring on the surface of the insulation material during electrical discharges often cause electrochemical oxidation of the insulation material.
The effect of this electrochemical oxidation is to generate polar chemical species, such as carboxylic acids for example, on the surface of the polymeric insulation material. This results in the surface of the insulation material becoming less hydrophobic and more hydrophilic. Therefore water condensing on the surface of the insulation material will tend to wet the surface more readily, thereby causing higher leakage currents and greater electrical discharge activity. This of course leads to the production of more corrosive by-products which attack the surface of the insulation material, degrading it further. In moist conditions, the insulation material is therefore subject to a cycle of high leakage currents, electrical discharges, generation of corrosive by-products, and electrochemical degradation.
U.S. Pat. No. 4,189,392 (Raychem) discloses an electrical insulation material which comprises a polymeric material (which includes an anti-tracking filler) having an erosion-inhibiting, hydrophobic, non-ionic, fluorine-substituted compound incorporated into it, The erosion-inhibiting compound has a fluorine-substituted aliphatic carbon chain linked to a group having an affinity for the polymeric material and/or the anti-tracking filler. The patent discloses an enormous range of fluorine-substituted compounds for incorporation into the polymeric material. The fluorine-substituted aliphatic carbon chain comprises at least four carbon atoms, and preferably comprises from 6 to 18 carbon atoms. The fluorine-substituted carbon chain is preferably a fluorinated alkyl group, e.g. a hexyl, heptyl, octyl, nonyl or decyl group. Preferably one, and more preferably two, carbon atoms immediately adjacent to the link with the group having an affinity for the polymeric material and/or the anti-tracking filler is an unsubstituted xe2x80x94CH2xe2x80x94 group. The compound may have more than one fluorine-substituted aliphatic carbon chain, e.g. two such chains linked to a divalent group having an affinity for the polymeric material and/or the anti-tracking filler.
The fluorine-substituted carbon chain of U.S. Pat. No. 4,189,392 may be linked directly to the group having an affinity for the polymeric material and/or the anti-tracking filler, or it may be linked through an alkoxy, acyl, amide or ester linkage. Ester linkages are particularly preferred. The nature of the group having an affinity for the polymeric material and/or the anti-tracking filler depends upon the polymeric material into which the fluorine-substituted compound is to be incorporated and the filler used. Where the group has an affinity for the polymeric material, it may contain atomic groupings similar to those in the polymeric material. The patent states that where the polymeric material comprises a polyolefin or an olefin copolymer, the group having an affinity for the polymeric material may comprise a carbon chain containing up to about 100 carbon atoms, preferably 3 to 20 carbon atoms. When the polymeric material comprises an acrylate or methacrylate polymer, the group preferably contains polar moieties, for example ester, ether or amide groups. Similarly for epoxy resins, polyurethanes, polyethers and epichlorohydrin rubbers, groups containing epoxide, urethane or ether linkages may be used as appropriate. When the polymeric material comprises a silicone polymer, the group preferably contains one or more silicon atoms bonded to oxygen, carbon halogen, nitrogen or hydrogen, for example the group may contain one or more siloxane linkages, or a polysiloxane chain containing up to about 40, and preferably from 4 to 30, siloxane linkages. The group having an affinity for the polymeric material may comprise more than one of the groups and linkages listed above if desired.
As already mentioned, U.S. Pat. No. 4,189,392 refers to a very wide range of fluorine-substituted compounds which may be used. The patent also discloses a very large number of suitable specific compounds. One of these many compounds which is mentioned is perfluoro telomer azelate (MPD3712 manufactured by Du Pont). The patent states that this is believed to be the azelaic acid ester of RfCH2CH2OH where Rf is mainly C4-C6 perfluoroalkyl.
Earlier in the present specification, it was stated that in moist conditions, high voltage polymeric insulation material is caused to degrade over time due to the cycle of high leakage currents, electrical discharge activity and attack by corrosive by-products. In practice, high voltage installations will not normally be subject to moisture condensation all of the time. Normally, such condensation will occur only during wet or humid weather, or overnight when the ambient temperature may fall below the dew point. After dawn during dry weather, the temperature may rise above the dew point, allowing the insulation material to dry out. The inventors of the present invention have found that if, during these dry periods, the insulation material regains its hydrophobicity (i.e. its ability to repel water), the gradual degradation of the material may be dramatically slowed down.
The inventors of the present invention have discovered that a specific, selected, small range of fluorine-substituted compounds, when incorporated as an additive into a polymeric insulation material, unexpectedly endows the insulation material with dramatically superior hydrophobicity retention and recovery (and consequent leakage current suppression under moist conditions), in comparison with previously used additive compounds.
According to a first aspect, the invention provides an electrical insulation material which is suitable for high voltage applications, comprising:
a) a polymeric material, and
b) an additive incorporated into the polymeric material, the additive comprising a compound of the formula:
Rfxe2x80x94O2C(CH2)xCO2xe2x80x94Rxe2x80x2f
where Rf and Rxe2x80x2f are fluorine-substituted aliphatic chains, and x is an integer in the range 9 to 18.
According to a second aspect, the invention provides a compound suitable for use as an additive in a polymeric electrical insulation material which is suitable for high voltage applications, the compound having the formula:
Rfxe2x80x94O2C(CH2)xCO2xe2x80x94Rxe2x80x2f
where Rf and Rxe2x80x2f are fluorine-substituted aliphatic carbon chains, and x is an integer in the range 9 to 18.
According to a third aspect, the invention provides the use of a compound of the formula:
Rfxe2x80x94O2C(CH2)xCO2xe2x80x94Rxe2x80x2f
where Rf and Rxe2x80x2f are fluorine-substituted aliphatic carbon chains, and x is an integer in the range 9 to 18, as an additive in a polymeric electrical insulation material which is suitable for high voltage applications.
It is well known that surface hydrophobicity may be determined by means of contact angle measurements. (The larger the contact angle, the more hydrophobic is the surface.) Furthermore, it is now well accepted that so-called static or equilibrium contact angle measurements, apply only to the theoretical ideal surface, i.e. one that is atomically smooth and completely homogeneous (chemically and physically). In practice, of course, no surface is perfectly smooth and homogeneous. For real surfaces, it has been found that the so-called dynamic receding contact angle is a more reliable measure of hydrophobicity than is the static contact angle. This is believed to be because the most important aspect of surface hydrophobicity (at least in relation to high voltage insulation materials) is the manner in which water runs off the surface of the material. If a drop of water leaves a long trail of water which has xe2x80x9cwetted-outxe2x80x9d on the surface of the material as it runs down the surface, then the surface is clearly not very hydrophobic. If, on the other hand, a running drop of water leaves substantially no water trail, then the surface is clearly hydrophobic. A good (and repeatable) measure of whether or not a drop would leave a wet trail on a surface is the dynamic receding contact angle of a sessile drop on the surface. This is the contact angle of the drop measured while the liquid of the drop is being drawn up into a syringe or pipette at a constant rate. If the contact angle is small (say below 40xc2x0-50xc2x0), then the surface is not very hydrophobic, but if it is large (e.g. 40xc2x0-50xc2x0 or above) then it is hydrophobic.